The present invention relates generally to data transmission via optical fibers, and specifically relates to reducing a tilt of a spectrum of light transmitted via an optical fiber.
The use of optical fibers to transmit voice, television, and computer data is becoming more widespread because of the advantages optical fibers provide over other data transmission technologies. Optical fibers transmit data at the speed of light, offer large bandwidths, and are less susceptible to external noise than other transmission media. It is also difficult to tamper with optical fibers by monitoring or injecting data in the middle of a connection; hence, optical fibers are appropriate for secure communications.
To increase the effectiveness of data transmission, multiple wavelengths of light may be transmitted along the same optic fiber. These wavelengths are combined by a wavelength division multiplexer and sent as a single transmitted signal. At the destination, the transmitted signal may then be separated into its component wavelengths or xe2x80x9cchannelsxe2x80x9d by a wavelength division demultiplexer.
Systems employing wavelength division multiplexers, however, are subject to a problem known as signal spectrum tilt. This tilt of the signal spectrum arises because of stimulated Raman scattering between photons being transmitted in the optical fiber and particles in the fiber. Due to the wavelength dependence of stimulated Raman scattering, system power is transferred from the lower wavelength channels to higher wavelength channels. This transfer of power results in a tilt, T, given by   T  =      {                                                      s              max                        -                          s              min                                                                                          if                ⁢                                  xe2x80x83                                ⁢                                  λ                  max                                            -                              λ                min                                       greater than             0                                                                          s              min                        -                          s              max                                                                                          if                ⁢                                  xe2x80x83                                ⁢                                  λ                  max                                            -                              λ                min                                      ≤            0                              
where the maximum, smax, and the minimum, smin, of the power spectrum occurs at a wavelength of xcexmax and xcexmin, respectively. In general, a tilt of a spectrum can be a positive number, in which case the sign of the tilt is positive, or a negative number, in which case the sign of the tilt is negative.
A power spectrum 10 with a positive tilt is shown in FIG. 1. Such a tilt may arise because of stimulated Raman scattering between photons and particles in the fiber. Also shown for comparison is a baseline 18 that corresponds to a power spectrum with no tilt. The positive tilt shown in FIG. 1 arises because power is transferred to the higher wavelengths at the expense of power in the lower wavelengths. Such a tilt implies that power is not transmitted uniformly across various channels. The non-uniformity of power distribution results in inefficient data transmission. For example, a receiver of a signal transmitted via the fiber may be configured to process data that uniformly occupies a certain set of channels. If the channels are not so occupied, ineffective data processing may occur.
For the foregoing reasons, there exists in the art a need for systems and methods for reducing a signal spectrum tilt in data transmission via optical fibers. For many applications, the ideal spectrum is one having zero tilt where power is distributed equally among all the channels.
The objective of reducing a first tilt of a power spectrum of light arising from scattering by an optical fiber can be achieved according to the present invention by adjusting a gain spectrum of an amplifier in the fiber transmission system. An amplifier is coupled to the fiber, and is configured to produce a gain spectrum with a second tilt that has a sign opposite to that of the first tilt. The first tilt, arising from scattering in the fiber, and the second tilt of opposite sign, arising from the configured amplifier, cancel to yield a power spectrum with a reduced tilt.
In particular, a method of reducing a tilt of a spectrum of light transmitted via an optical fiber is presented herein including transmitting a light signal over the optical fiber to a receiver, wherein a power spectrum of the light signal at the receiver has a tilt because of scattering. The method also includes configuring the amplifier in the transmission system to compensate for the tilt and thereby reduce the tilt of the power spectrum of the light signal.
A method for reducing a tilt of a spectrum of light transmitted via an optical fiber is also presented below that includes providing a source of light, such as a laser, and connecting the source to a receiver of light via an optical fiber. The light populating a set of channels that is sent from the source to the receiver via the fiber has a first power spectrum at the receiver having a first tilt with a first sign. In addition, the method includes connecting at least one of the source, receiver, and fiber with an amplifier that is configured to produce a gain spectrum with a second tilt that has a second sign that is opposite to the first sign. The choice of second tilt depends on the set of channels, such as, for example the number of channels in the set. The method also includes allowing a packet of light, populating the set of channels and travelling from the source to the receiver, to pass the amplifier. This results in a second power spectrum of the packet of light at the receiver that has a residual tilt whose magnitude is smaller than a magnitude of the first tilt.
In the method, if the set of channels corresponds to a full C- band and a full L-band, then the amplifier is configured to produce a gain spectrum with a second tilt lying in the interval (xe2x88x921.1T, xe2x88x920.9T) where T is the first tilt. If, on the other hand, the set of channels corresponds to a random population in a C-band and an L-band, then the amplifier is configured to yield a gain spectrum in the C-band with a tilt lying in the interval (xe2x88x920.83T, xe2x88x920.63T), and a gain spectrum in the L band with a tilt lying in the interval (xe2x88x921.1T, xe2x88x920.9T), where T is the first tilt.
A method is also presented below for reducing a tilt of a spectrum of light transmitted via an optical fiber. The method includes sending a first packet of light from a source to a receiver via the optical fiber, and, at the receiver, measuring a spectrum of the light, the spectrum having a first tilt with a first sign. The method further includes configuring an amplifier for producing a gain spectrum with a second tilt that has a second sign that is opposite to the first sign, and coupling the amplifier to the optical fiber. The method also includes allowing a second packet of light to travel from the source to the receiver by passing the amplifier, wherein a second power spectrum of the second packet of light at the receiver has a residual tilt whose magnitude is smaller than a magnitude of the first tilt.
Also presented below is an apparatus for reducing a tilt of a spectrum of light transmitted via an optical fiber. The apparatus includes a source of light and an optical fiber. The fiber is capable of receiving light at a first end of the fiber, and emitting scattered light at a second end of the fiber, the scattered light having a first spectrum with a first tilt characterized by a first sign. The apparatus further includes a connector for connecting the source of light to the first end of the fiber, and an amplifier configured to produce a gain spectrum that has a second tilt characterized by a second sign that is opposite to the first sign. The amplifier is coupled to the optical fiber so that light propagating from the first end to the second end passes therethrough. The light reaching the second end has a second spectrum with a residual tilt whose magnitude is smaller than a magnitude of the first tilt.